Why Reusing logic with hooks in React? - Purpose & Use Cases

The Big Idea

What if you could fix a bug once and watch it update everywhere instantly?

The Scenario

Imagine you have several React components that need to share the same behavior, like fetching data or handling a timer. You copy and paste the same code into each component.

The Problem

Copying code everywhere makes your app hard to maintain. If you find a bug or want to improve the logic, you must update every copy. This wastes time and causes mistakes.

The Solution

Hooks let you write the shared logic once and reuse it in many components. This keeps your code clean, easy to update, and consistent across your app.

Before vs After

✗ Before

function ComponentA() {
  useEffect(() => {
    // fetch data logic here
  }, []);
  // repeated in ComponentB
}

✓ After

import { useEffect } from 'react';

function useFetchData() {
  useEffect(() => {
    // fetch data logic here
  }, []);
}

function ComponentA() {
  useFetchData();
}
function ComponentB() {
  useFetchData();
}

What It Enables

You can build cleaner, more reliable React apps by sharing logic easily and avoiding repeated code.

Real Life Example

Think of a timer hook that counts seconds. Instead of writing timer code in every component, you write one hook and use it wherever you need a timer.

Key Takeaways

Copy-pasting logic causes bugs and wastes time.

Hooks let you share logic cleanly across components.

This makes your app easier to maintain and improve.

Practice

1. What is the main purpose of creating a custom hook in React?

easy

A. To create new HTML elements

B. To reuse stateful logic across multiple components

C. To style components with CSS

D. To replace React components entirely

Solution

1. Step 1: Understand what hooks do

   Hooks let you use React features like state and effects inside functions.

2. Step 2: Identify the role of custom hooks

   Custom hooks let you reuse this logic in many components without repeating code.

3. Final Answer:

   To reuse stateful logic across multiple components -> Option B

4. Quick Check:

   Custom hooks = reuse logic [OK]

Hint: Custom hooks start with 'use' and share logic [OK]

Common Mistakes:

• Thinking hooks create UI elements
• Confusing hooks with styling tools
• Believing hooks replace components

2. Which of the following is the correct way to define a custom hook in React?

easy

A. function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; }

B. function counterHook() { const [count, setCount] = useState(0); return [count, setCount]; }

C. function UseCounter() { const [count, setCount] = useState(0); return [count, setCount]; }

D. function counter() { const [count, setCount] = useState(0); return [count, setCount]; }

Solution

1. Step 1: Recall naming rules for custom hooks

   Custom hooks must start with 'use' to follow React conventions and enable linting.

2. Step 2: Check function names

   Only function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; } starts with 'use' and uses camelCase correctly.

3. Final Answer:

   function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; } -> Option A

4. Quick Check:

   Custom hook names start with 'use' [OK]

Hint: Custom hooks always start with 'use' prefix [OK]

Common Mistakes:

• Not starting function name with 'use'
• Using uppercase first letter in hook name
• Naming hooks like regular functions

3. Given this custom hook code, what will be logged when ExampleComponent renders?

function useToggle(initial) {
  const [on, setOn] = React.useState(initial);
  const toggle = () => setOn(!on);
  return [on, toggle];
}

function ExampleComponent() {
  const [isOn, toggle] = useToggle(false);
  React.useEffect(() => {
    console.log(isOn);
  }, [isOn]);
  return Toggle;
}

medium

A. undefined on first render, then false after button click

B. true on first render, then false after button click

C. No output because console.log is inside useEffect

D. false on first render, then true after button click

Solution

1. Step 1: Analyze initial state in useToggle

   The hook starts with initial = false, so isOn is false initially.

2. Step 2: Understand useEffect logging

   useEffect logs isOn after render; initially logs false, after toggle logs true.

3. Final Answer:

   false on first render, then true after button click -> Option D

4. Quick Check:

   Initial state false, toggles true [OK]

Hint: Initial state logs first, toggle changes it [OK]

Common Mistakes:

• Assuming initial state is true
• Thinking console.log won't run inside useEffect
• Confusing toggle function behavior

4. What is wrong with this custom hook code?

function useCounter() {
  const [count, setCount] = React.useState(0);
  function increment() {
    count = count + 1;
  }
  return { count, increment };
}

medium

A. Directly modifying count variable instead of using setCount

B. Missing React import for useState

C. increment function should return count

D. Custom hook name should not start with 'use'

Solution

1. Step 1: Check how state is updated

   State variables like count must be updated via setCount, not by direct assignment.

2. Step 2: Identify the error in increment

   increment tries to assign count = count + 1, which does not update React state.

3. Final Answer:

   Directly modifying count variable instead of using setCount -> Option A

4. Quick Check:

   State updates need setCount [OK]

Hint: Always use setState to update state, never assign directly [OK]

Common Mistakes:

• Trying to assign state variable directly
• Forgetting to call setCount
• Confusing variable assignment with state update

5. You want to create a custom hook useFetch that fetches data from a URL and returns loading state, data, and error. Which approach correctly reuses logic and handles updates safely?

function useFetch(url) {
  const [data, setData] = React.useState(null);
  const [loading, setLoading] = React.useState(true);
  const [error, setError] = React.useState(null);

  React.useEffect(() => {
    setLoading(true);
    fetch(url)
      .then(res => res.json())
      .then(json => {
        setData(json);
        setLoading(false);
      })
      .catch(err => {
        setError(err.message);
        setLoading(false);
      });
  }, [url]);

  return { data, loading, error };
}

hard

A. This hook will cause infinite loops because setState is inside useEffect

B. This hook should not use useEffect for fetching data

C. This hook correctly reuses fetch logic and updates state on URL change

D. This hook must return an array, not an object

Solution

1. Step 1: Check useEffect dependency

   useEffect runs when URL changes, so fetch updates correctly.

2. Step 2: Verify state updates inside fetch

   State updates happen after fetch resolves or errors, no infinite loops occur.

3. Step 3: Confirm return value format

   Returning an object with data, loading, error is a common pattern and valid.

4. Final Answer:

   This hook correctly reuses fetch logic and updates state on URL change -> Option C

5. Quick Check:

   useEffect fetches on URL change, updates state [OK]

Hint: useEffect with URL dependency safely fetches data [OK]

Common Mistakes:

• Thinking setState inside useEffect causes loops
• Not using useEffect for side effects
• Confusing return types of hooks